Bug#388845: debian-installer: Warn of sarge apt-proxy when selecting mirror

[Wolfgang Köbler <wolfgang@koebler.com>]

Of course you could display the incompatibility hint when the connection
times out (instead of when selecting a mirror). Then it would only be seen by
people that have a problem (assuming they have the patience to wait for the
timeout). On the other hand it is nice to see a hint before the problem is
there.

Or you could say that a connection timeout is a good hint that your proxy is
broken so that you do not need any further hints. The problem about the
apt-proxy bug is that it is a bit hard to detect:
- AFAIR some data is successfully retrieved from the apt-proxy before
  installation fails.
- I could switch to the d-i console via Alt-F2 and download the file
  displayed in the errormessages on Alt-F4.
So my apt-proxy mirror seemed to be ok. (Of course it was not.)